@SteamLouis-- Have you tried using booklets or stencils to help you? Both are available, they're sometimes sold alone, other times they are sold in mehndi kits along with henna pens or henna powder.
I've gotten quite good at mehndi designs. It is tough to draw on my right hand with the left but I'm getting used to that as well. I prefer the Indian designs. I think they are more beautiful and there are more designs to choose from.
When I started out though, I had trouble. It was a matter of using the right henna pens and practicing. I bought some booklets with example designs to learn from. I also used stencils in the very beginning. I suggest starting with these. Once you have a good control on applying the henna, you can start developing designs off the top of your head which is what professional mehndi artists too. But it requires a lot practice. There are usually enough girls at parties and events more than happy to get a free mehndi design on their hand. It's great for practice.